Ferritic stainless steel

ABSTRACT

Provided is a ferritic stainless steel including, as a ferritic stainless steel used in a separator for a fuel cell, a base material including, in weight %, C: 0.003% to 0.012%, N: 0.003% to 0.015%, Si: 0.05% to 0.15%, Mn: 0.3% to 0.8%, Cr: 20% to 24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7%, Ti: 0.03% to 0.1%, and the remainder being Fe and inevitable impurities. A first scale layer including chromium oxide is formed on a surface of the base material, and a second scale layer including chromium oxide and manganese oxide is formed on a surface of the first scale layer. A silicon content included in each of the first scale layer and the second scale layer is 0.2 weight % or less, and the following formula is satisfied: Nb+Mn≧8Si where Nb, Mn and Si are weight % amounts of corresponding components, respectively.

TECHNICAL FIELD

The present disclosure relates to a ferritic stainless steel, and inparticular, to a ferritic stainless steel capable of maintaining highconductivity under a high temperature oxidizing environment.

BACKGROUND ART

Stainless steel has excellent corrosion resistance and oxidationresistance, and thereby has been used in various fields from roomtemperature to high temperature. Among these, extensive studies formanufacturing components such as a separator of a fuel cell operatedunder a high temperature environment using stainless steel have beenongoing.

In order to use stainless steel in a high temperature fuel cell, athickness of a scale formed on a surface of the stainless steel shouldnot excessively increase or electrical conductivity should not bereduced under a high temperature oxidizing environment. When the scalethickness increases by a certain level or higher, the scale is peeledoff damaging a material, and when electrical conductivity is low, fuelcell efficiency may decrease.

Accordingly, such properties need to be fulfilled to use stainless steelas a fuel cell component.

When stainless steel is oxidized, chromium oxide (Cr₂O) is formed on thesurface, and corrosion resistance is obtained due to an oxidized scaleformed with such chromium oxide. However, the scale formed at the timehas low electrical conductivity while having excellent corrosionresistance. In addition, general stainless steel includes silicon in acertain amount, which causes a problem of exhibiting an insulatingeffect by forming silicon oxide at an interface between the stainlesssteel and the scale. The image of the scale famed with chromium oxide asabove is shown in FIG. 3, the image of silicon oxide formed is shown inFIG. 4, and a result of analyzing components of a layer with clusters ofsilicon oxide is shown in FIG. 5.

Technologies of adding rare earths or controlling a siliconconcentration to be very low in stainless steel have been developed inview of the above problems, however, such technologies are difficult touse in common mass production-type metal manufacturing processes, andmanufacturing costs excessively increase.

Accordingly, development of stainless steel preventing silicon oxideformation and having high electrical conductivity even at a hightemperature has been required.

DISCLOSURE Technical Problem

The present disclosure has been made in view of the above, and thepresent disclosure is directed to providing a ferritic stainless steelcapable of maintaining high electrical conductivity even under a hightemperature oxidizing environment.

Technical Solution

In view of the above, a ferritic stainless steel according to oneembodiment of the present disclosure includes, as a ferritic stainlesssteel used in a separator for a fuel cell, a base material including, inweight %, C: 0.003% to 0.012%, N: 0.003% to 0.015%, Si: 0.05% to 0.15%,Mn: 0.3% to 0.8%, Cr: 20% to 24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7%,Ti: 0.03% to 0.1%, and the remainder being Fe and inevitable impurities,wherein, when exposed to an oxidizing environment of 300° C. to 900° C.,a first scale layer including chromium oxide is formed on a surface ofthe base material, a second scale layer including chromium oxide andmanganese oxide is formed on a surface of the first scale layer, asilicon content included in each of the first scale layer and the secondscale layer is 0.2 weight % or less, and the following formula issatisfied.

Formula: Nb+Mn≧8Si (Nb, Mn and Si are weight % amounts of correspondingcomponents, respectively.)

A thickness of the second scale layer is ⅔ or greater of a thickness ofthe whole scale layer.

A third scale layer including niobium oxide is formed between the basematerial and the first scale layer.

Advantageous Effects

According to a ferritic stainless steel of the present disclosure,components capable of maintaining high electrical conductivity over along period of time even when used in a separator of a fuel cell and thelike under a high temperature oxidizing environment can be manufactured.

DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional TEM image of a ferritic stainless steel accordingto one embodiment of the present disclosure.

FIG. 2 shows EDS graphs of components of a first scale layer formed withchromium oxide and a second scale layer formed with chromium/manganeseoxide.

FIG. 3 is a sectional TEM image of a comparative example forming only achromium oxide layer.

FIG. 4 is a sectional TEM image of a comparative example forming asilicon oxide layer between a base material and a scale.

FIG. 5 is an EDS graph of components of the silicon oxide layer formedbetween the base material and the scale.

FIG. 6 is a graph showing and comparing silicon fractions in an exampleof the present disclosure and a comparative example depending on adepth.

FIG. 7 is a graph showing a niobium fraction in an example of thepresent disclosure depending on a depth.

MODE FOR DISCLOSURE

The terminology used herein is for describing particular embodimentsonly and is not intended to be limiting of the present inventiveconcept. Singular forms used herein include plural forms as well, unlessthe context clearly indicates otherwise. The meaning of “include” usedin the specification specify the presence of stated specific features,areas, integers, steps, operations, elements, and/or components thereof,but do not preclude the presence or addition of one or more otherspecific features, areas, integers, steps, operations, elements,components and/or groups thereof.

Unless otherwise specified, all the terms including technical terms andscientific terms used herein have the same meanings commonlyunderstandable to those skilled in the art relating to the presentdisclosure. In addition, the terms defined in generally useddictionaries are additionally interpreted to have meanings correspondingto relating scientific literatures and contents disclosed now, and arenot interpreted either ideally or very formally unless definedotherwise.

Hereinafter, a ferritic stainless steel according to preferredembodiments of the present disclosure will be described with referenceto the accompanying drawings.

The present disclosure relates to a ferritic stainless steel including,with iron as a matrix structure, C: 0.003% to 0.012%, N: 0.003% to0.015%, Si: 0.05% to 0.15% or less, Mn: 0.3% to 0.8%, Cr: 20% to 24%,Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7% Ti: 0.03% to 0.1% (hereinbefore,weight %), and satisfying the following formula.

Nb+Mn≧8Si   Formula:

Nb, Mn and Si are weight % amounts of corresponding components,respectively.

The above-mentioned formula limits the content of the manganese and thecontent of the niobium are greater than the content of the silicon by acertain level or higher, and exhibits a composition required forpreventing silicon oxide formation. Manganese and niobium has highoxidization rate and diffusion rate, and are formed as oxides on anexternal surface layer of a scale or at an interface between a basematerial and the scale, and as a result, oxide production occurring fromsilicon oxidation may be prevented. Such an effect may not be expectedwhen the manganese and the niobium are included at a certain level orlower compared to silicon, and therefore, satisfying the range of theabove-described formula is important.

Hereinafter, reasons for limiting the range of each component will bedescribed. Moreover, % described below all means weight %.

Carbon (C) is an element essentially included in a stainlessmanufacturing process. When a carbon content excessively increases,precipitates such as chromium carbide are formed, which may adverselyaffect base material composition and oxidation characteristics, andtherefore, the upper limit is limited to 0.013%. However, limiting thecarbon content to be extremely low causes an excessive increase in thecosts and therefore, the lower limit is preferably limited to 0.003%.

When the nitrogen (N) content excessively increases, various nitridesare precipitated, or pores are produced adversely affecting productqualities, and therefore, the upper limit is limited to 0.015%. However,limiting the nitrogen content to be extremely low causes an excessiveincrease in the costs and therefore, the lower limit is preferablylimited to 0.008% or greater.

Silicon (Si) forms an insulator film by forming film-type precipitatesat an interface between a scale and a base material when the material isexposed to a high temperature, and is a component that needs to bestrictly limited, and therefore, the upper limit is limited to 0.15%.However, in order to reduce the silicon content to 0.05% or lower,high-cost processes such as vacuum melting need to be carried out, andtherefore, the lower limit is limited to 0.05% in the presentdisclosure.

Manganese (Mn) is quickly diffused when stainless steel is oxidized at ahigh temperature to form dense manganese/chromium oxide on an externallayer of a scale, and therefore, needs to be added in 0.3% or greater.However, an excessive addition of manganese excessively facilitatesscale growth causing a concern of scale peel-off, and therefore, theupper limit is limited to 0.8%.

Chromium (Cr) is an essential element for securing corrosion resistanceof stainless steel. In order to prevent chromium exhaustion caused froman oxidation over a long period of time under a high temperatureoxidizing environment, a minimum of 20% or greater thereof needs to beadded. However, the upper limit is preferably limited to 24% in orderfor preventing an increase in the manufacturing costs, and precipitationof chromium carbide, intermetallic compounds and the like.

Molybdenum (Mo) is an element capable of increasing material strengthunder a high temperature environment. Accordingly, a minimum of 0.1% orgreater thereof needs to be added, however, when considering thatmolybdenum is a high-priced element, the upper limit is preferablylimited to 0.4% for suppressing an increase in the manufacturing costs.

Niobium (Nb) forms an oxide by being oxidized at a scale/base materialinterface due to its excellent oxidation characteristics, and suppressesformation of an insulating silicon oxide therethrough, and therefore,needs to be added in 0.1% or greater. Meanwhile, when added in excess,hot workability is inhibited and manufacturing costs increase, andtherefore, the upper limit is preferably limited to 0.7%.

Titanium (Ti) increases material strength by forming an internal oxideright below an interface between a base material and a scale, that is,near a surface of the base material, at a high temperature, andtherefore, the content of 0.03% or greater is required. However, anexcessive addition causes an increase in the manufacturing costs andforms titanium oxide on the outside of the scale, and therefore, theupper limit is preferably limited to 0.1%.

When such a ferritic stainless steel is exposed to an oxidizingenvironment of 300° C. to 900° C., a first scale layer includingchromium oxide is formed on a surface of the ferritic stainless steel,and a second scale layer including chromium oxide and manganese oxide isformed on a surface of the first scale layer, wherein a thickness of thesecond scale layer is ⅔ or greater of a thickness of the whole scalelayer.

As shown in FIG. 1, there is a difference in the thickness between thefirst scale layer including chromium oxide and the second scale layerincluding chromium oxide and manganese oxide. Chromium oxide has lowelectrical conductivity and is not suitable to be used as a component ofa fuel cell, however, manganese oxide has relatively high electricalconductivity and may be used as a component of a fuel cell. In order tohave required electrical conductivity, the thickness of the second scalelayer needs to be larger than the thickness of the first scale layer,and the thickness of the second scale layer needs to be larger by atleast two times or greater than the thickness of the first scale layer.Accordingly, of the whole scale layer, the second scale layer preferablyhas a thickness of ⅔ or greater. In addition, as shown in FIG. 2, thesecond scale layer include manganese, chromium and the like, and thefirst scale layer includes chromium and the like.

Between the ferritic stainless steel and the first scale layer, a thirdscale layer including niobium oxide is preferably formed.

Between a base material, that is, stainless steel, and a scale layerformed on a surface thereof, readily oxidizable silicon normally formsan oxide layer. An image of such silicon oxide layer formation is shownin FIG. 4. Silicon oxide has extremely low electrical conductivity andmay not be used as a component for a fuel cell. Accordingly, productionof silicon oxide needs to be suppressed by forming, instead of silicon,an oxide having high electrical conductivity while being oxidized fasterthan silicon. For this, niobium is added to form niobium oxide betweenthe base material and the scale layer in the present disclosure, andsilicon oxide formation is capable of being suppressed. More preferably,silicon oxide production needs to be completely prevented, however,completely suppressing the production of silicon oxide is verydifficult. When niobium oxide is produced, an opportunity of siliconoxidation is reduced as much, and accordingly, total silicon oxideproduction may be reduced, and a decrease in the electrical conductivitymay be prevented therefrom.

FIG. 6 shows a graph comparing silicon fractions in an example of thepresent disclosure and a comparative example forming a silicon oxidelayer depending on a depth. According to FIG. 6, it is seen that asilicon fraction appears to be high at depths of 1 μm to 5 μm in acomparative example, however, a silicon fraction does not increase inthe same range in an example of the present disclosure.

Meanwhile, as shown in FIG. 7, a niobium content appears to be high atdepths of 2 μm to 5 μm in an example of the present disclosure.

Hereinafter, compositions, whether the formula is satisfied or not, andwhether silicon oxide is produced or not in examples of the presentdisclosure and comparative examples are compared in Table 1.

TABLE 1 Silicon Steel Type C N Si Mn Cr Mo Nb Ti Formula Oxide Example 10.005 0.007 0.11 0.5 21.3 0.15 0.43 0.05 Satisfied Not Produced Example2 0.009 0.004 0.14 0.6 22.6 0.22 0.72 0.08 Satisfied Not ProducedExample 3 0.007 0.013 0.06 0.4 23.5 0.33 0.65 0.04 Satisfied NotProduced Example 4 0.011 0.006 0.08 0.7 23.3 0.11 0.25 0.04 SatisfiedNot Produced Example 5 0.007 0.009 0.09 0.5 20.5 0.25 0.53 0.07Satisfied Not Produced Comparative 0.001 0.008 0.12 0.4 22.3 0.2 0.150.08 Not Produced Example 1 Satisfied Comparative 0.008 0.007 0.12 0.122.6 0.23 0.7 0.05 Not Produced Example 2 Satisfied

As shown in Table 1, it was seen that silicon oxide is formed greatlyreducing electrical conductivity when the composition or the formula ofthe present disclosure is not satisfied.

Hereinbefore, embodiments of the present disclosure have been describedwith reference to the accompanying drawings, however, it is to beunderstood that those having common knowledge in the art to which thepresent disclosure belongs may implement the present disclosure in otherspecific forms without modifying technological ideas or essentialcharacteristics of the present disclosure.

Therefore, embodiments described above need to be understood asillustrative rather than limitative in all aspects. The scope of thepresent disclosure is represented by the attached claims rather than thedetailed descriptions provided above, and the meaning and the scope ofthe claims, and all modifications or modified formed deduced fromequivalent concepts thereof need to be interpreted as being included inthe scope of the present disclosure.

1. A ferritic stainless steel used for a separator for a fuel cell,comprising a base material including, in weight %, C: 0.003% to 0.012%,N: 0.003% to 0.015%, Si: 0.05% to 0.15%, Mn: 0.3% to 0.8%, Cr: 20% to24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7%, Ti: 0.03% to 0.1%, and theremainder being Fe and inevitable impurities, wherein a first scalelayer including chromium oxide is formed on a surface of the basematerial, and a second scale layer including chromium oxide andmanganese oxide is formed on a surface of the first scale layer, asilicon content included in each of the first scale layer and the secondscale layer is 0.2 weight % or less, and the following formula issatisfied: formula: Nb+Mn≧8Si (Nb, Mn and Si are weight % amounts ofcorresponding components, respectively).
 2. The ferritic stainless steelof claim 1, wherein a thickness of the second scale layer is ⅔ orgreater of a thickness of the whole scale layer.
 3. The ferriticstainless steel of claim 2, comprising a third scale layer includingniobium oxide formed between the base material and the first scalelayer.